In recent years, widely used in motor vehicle air conditioners in place of conventional serpentine condensers are condensers which comprise, as shown in FIG. 16, a pair of headers 110, 111 arranged in parallel and spaced apart from each other, parallel flat refrigerant flow tubes 112 made of aluminum and each joined at its opposite ends to the two headers 110, 111, corrugated aluminum fins 113 each disposed in an air flow clearance between the adjacent refrigerant tubes 112 and brazed to the adjacent tubes, an inlet pipe 114 connected to the upper end of peripheral wall of the first 110 of the headers, an outlet pipe 115 connected to the lower end of peripheral wall of the second 111 of the headers, a first partition 116 provided inside the first header 110 and positioned above the midportion thereof, and a second partition 117 provided inside the second header 111 and positioned below the midportion thereof, the number of refrigerant tubes 112 between the inlet pipe 114 and the first partition 116, the number of refrigerant tubes 112 between the first partition 116 and the second partition 117 and the number of refrigerant tubes 112 between the second partition 117 and the outlet pipe 115 decreasing from above downward to provide groups of channels. A refrigerant flowing into the inlet pipe 114 in a vapor phase flows zigzag through units of passage groups in the condenser before flowing out from the outlet pipe 115 in a liquid phase. The condensers of the construction described (see JP-B No. 45300/1991) are called multiflow condensers, and realize high efficiencies, lower pressure losses and supercompactness.
It is required that the refrigerant flow tube 112 of the condenser described be excellent in heat exchange efficiency and have pressure resistance against the high-pressure gaseous refrigerant to be introduced thereinto. Moreover, the tube needs to be small in wall thickness and low in height so as to make the condenser compact.
Such a flat tube for use as a refrigerant flow tube is known as disclosed, for example, in U.S. Pat. No. 5,553,377. The disclosed flat tube comprises a pair of flat walls opposed to each other, two side walls interconnecting opposite side edges of the two flat walls, and a plurality of reinforcing walls interconnecting the flat walls, extending longitudinally of the tube and spaced apart from one another by a predetermined distance. The flat tube is made from a metal plate which has two flat wall forming portions connected together by a joint portion, a plurality of reinforcing wall forming portions upwardly projecting from each of the wall forming portions integrally therewith, and side wall forming portions formed respectively at opposite side edges of the plate and upwardly projecting therefrom integrally therewith, by bending the metal plate into the form of a hairpin at the joint portion by the roll forming process. The flat walls are formed by the flat wall forming portions, and one of the side walls is formed by the joint portion. The reinforming wall forming portions and the side wall forming portion on one of the flat wall forming portions are brazed to those on the other flat wall forming portions in corresponding relation end-to-end, whereby the reinforcing walls and the other side wall are formed.
When the metal plate is bent in the form of a hairpin by roll forming in producing the conventional flat tube, the reinforcing wall forming portions, as well as the side wall forming portions, are joined together in corresponding pairs without displacement, whereas when a pressure acts on the plate during brazing to bring the two flat wall forming portions toward each other, it is likely that the reinforcing wall forming portions in each pair, as well as the side wall forming portions, will be displaced from each other widthwise of the flat wall forming portions. If the plate is brazed in this state, the length or area of the joint to be formed between each corresponding pair or the side wall forming portions will become smaller or no joint will be formed locally, entailing the problem of seriously impaired pressure resistance or leakage of the fluid flowing through the flat tube.
An object of the present invention is to overcome the above problem, to provide a metal plate for use in producing a flat tube which makes it possible to join the corresponding pairs of reinforcing wall forming portions, as well as the side wall forming portions, without displacement after the plate is bent into the shape of a hairpin, and to provide the flat tube and a process for producing the flat tube.